Although most asphalts are used in paving, a significant percentage are used for other applications, primarily roofing and specialty coatings. Asphalts for roofing and specialty coatings are air-blown to reduce aging and to increase weather resistance. The air-blowing process also increases the usefulness of the asphalt by raising the softening point from a typical starting point of about 40.degree. C. to a product having a softening point of 80.degree. C. or higher. Unblown asphalts have low softening points, low viscosity (e.g., 12 cps at 205.degree. C.) and high penetration (e.g., 250-300 mm/10 at 25.degree. C.). In order to be able to generate an asphalt that has the desirable properties for roofing applications, it is necessary to air-blow it. The air-blowing process, also referred to as the oxidizing process, is used to thicken the asphalt raw material. As a result, the penetration decreases, and both the softening point and viscosity increase. Nevertheless, the resultant blown product at a selected softening point can be too hard and brittle at certain cooler temperatures, or can be so soft that it will flow on a hot summer day. Consequently, the relationship between the viscosity, penetration and softening point is very important. The trend in worldwide sources of asphalt is that the asphalt raw material blows to a harder product and the sulfur content is steadily increasing. These asphalts have lower penetrations when blown to the desired softening point, and the product properties of the resultant asphalt are adversely affected.
The air-blowing process involves loading the unblown asphalt raw material in a converter at temperatures within a range of from about 150.degree. C. to about 205.degree. C. Air is bubbled or blown through the molten asphalt. The reaction produced by the blowing is exothermic and raises the temperature of the asphalt to a temperature of about 260.degree. C. The maximum temperature is usually controlled by a water-cooled jacket. The process is usually carried out in batches. The processing time can take from 1 hour up to about 18 hours for reaching the desired softening point. The amount of processing time is dependent on the process temperature and the characteristics of the asphalt. Catalysts are frequently blended into the mixture to increase the reaction rate, which thereby reduces the processing time. A commonly used catalyst is ferric chloride, which is typically used in concentrations of up to about 1 percent of the blended catalyst and asphalt. Catalysts can also help provide better penetrations for the desired softening point.
One of the problems with asphalt processing is the generation of unpleasant gaseous byproducts. Sulfur organic compounds are a significant component of these gaseous emissions. The asphalt blowing process produces flue gases including hydrocarbons, hydrogen sulfide, nitrogen oxide and carbon monoxide. The emissions of volatile organic compounds are controlled by best available control technology (BACT) incineration. As a result of the incineration, the oxidation of heterosulfur organic compounds and hydrogen sulfide generates SO.sub.x. Typical undesirable SO.sub.x emissions include sulfur oxides such as SO.sub.2 and SO.sub.3, and combinations of those compounds with other substances such as water.
Unfortunately, the combustion process has its own undesirable waste stream. SO.sub.x emissions are generated from the combustion of the hydrogen sulfide or by the oxidation of mercaptans and heavier sulfur-organic compounds in the incinerator. One process presently used to reduce SO.sub.x emissions involves cleaning the effluent with expensive caustic scrubber equipment. The scrubbers require extensive initial capital costs and significant annual operating costs. The caustic scrubbing process generates such undesirable byproducts as sodium and potassium sulfides, sulfites and sulfates, all of which require special handling for disposal.
It would be advantageous if a process could be developed to reduce SO.sub.x emissions without requiting high cost capital equipment. Such a process would ideally also not generate any undesirable byproducts. Further, such a process should not retard the asphalt blowing process or adversely affect the softening point/viscosity/penetration relationships in the blown asphalt.
NO.sub.x emissions are another unpleasant gaseous byproduct from asphalt processing. It would be desirable if a process could be developed to reduce SO.sub.x emissions without greatly increasing the amount of NO.sub.x emissions.
It would also be desirable if such a process produced an asphalt that is noncorrosive and does not cause any detrimental effect on roofing materials or their properties. By way of background, asphalts for roofing can be used as built up roofing asphalt (BURA) or in asphalt roofing shingles. On a built up roof, a layer of asphalt is applied to the roof, followed by a mat of fiberglass or organic material, followed by another layer of asphalt and then another mat. The asphalt comes into contact with various roofing materials by initial contact and leaching. Such roofing materials include metallic structures such as eaves, gutters, flashings and nails. Therefore, it can be seen that a process to reduce SO.sub.x emissions should also produce an asphalt that does not have a detrimental effect on roofing materials, so that the materials do not rust, corrode or lose their strength. The asphalt should also be able to maintain a lasting bond with the other materials with which it comes in contact, including both metallic and non metallic materials.